Thermionic amplifier



Dec. 10, 1940. ug-r 2,224,699

THERMIONIC AMPLIFIER, I

Filea Dec 15, 1956 2 Sheets-Sheet 1 IIVVV JlllAAllAl II I lNVENT OR v NOEL M. RUST ATTORNEY Deb. 10, 1940. N. M. RUST THERMIONIC AMPLIFIER 2 Sheets-Sheet 2 Filed Dec. 15, 1936 inn:-

Fig 13 INVENTOR NOEL M. RUST BY m,

ATTORNEY Patented Dec. 10, 1940 TENT FFIfiE THERMIONIC AMPLIFIER Noel Meyer Rust, Chelmsford, England, assignor to Radio Corporation of America, a corporation of Delaware Application December 15, 1936, Serial No. 115,913 In Great Britain December 16, 1935 9 Claims.

This invention relates to thermionic amplifiers and more particularly to gain control means therefor.

It is common practice at the present time to provide radio receivers with automatic gain control means in connection with one or more of the carrier frequency amplifiers thereof, such means being provided with a view to reducing or eliminating fading efiects. The most usual method of obtaining such anti-fading automatic gain control is to employ as the valve or valves whose gain is to be controlled, the so-called variable-mu type of valve and to utilise uni-directional potentials automatically varied in dependence upon received signal strength to supply gain controlling grid bias to the said variable-mu valve or valves.

It may be shown that. this method of effecting gain control, namely by varying the. bias upon a so-called variable-mu valve has serious limitations and is a common cause of serious distortion for it depends for its operation upon the curvature of the grid voltage-anode current characteristic of the valve controlled. Probably the most serious defect of this method of gain control is the interference introducedas a result of crossmodulation and it is a common experience that cross-modulation may be so serious as to prevent satisfactory reception of a desired weak station when there is an undesired strong station operating on a wave length which, though adjacent the wave length of the desired station, is so far away that it ought not to cause interference having regard to the selectivity of the stages following the gain controlled valve. For example in a superheterodyne receiver having an intermediate frequency amplifier of such selectivity that an undesired powerful station ought not, from the point of view of selectivity alone, to interfere with reception of a desired weak staticn, it is common experience that reception of the desired station is rendered practically impossible due to cross-modulation effects in the gain-controlled stage or stages. preceding the intermediate frequency amplifier for it will be appreciated that no sharpening of the selectivity of the intermediate frequency amplifier will elimihate such cross-modulation effects.

The main object of the present invention is to provide a gain control system which does not depend upon the curvature of the grid voltageanode current characteristic and which indeed does not depend upon any curvature of characteristic at all.

Accordingto this invention gain control is efinvention, to use as the said valve a valve havingv 10 2. constant-mu or a valve with a square law characteristic or whose mutual conductance variation is rectilinear with grid bias variation.

Mathematical analysis shows that the latter type of valve will produce far less cross-modulation 15.

than the variable-mu type of valve and in fact theoretically with a square law anode currentgrid voltage characteristic no cross modulation should be produced if the subsequent detection process is rectilinear. 20

In carrying out the present invention it "is preferred to employ so-called electron beam valves, 1. e. valves wherein the discharge partakes more of the nature of that in a cathode ray tube than of that in an ordinaryvalve as at present in common use.

In the drawings:

Fig. 1 shows a negative feedback circuit embodying a simpleform of the invention,

Figs. 2 to 4 inclusive illustrate respectively different modifications of the arrangement in Fig. 1,

Fig. 5 shows a modification using regenerative feedback,

Fig. 6 illustrates a modification employing both positive and negative feedback,'

Figs. 7, 8, 9 and 10 illustrate further respective ly dilferent modifications of the type of circuit in Fig. 1,

Fig. 11 shows a modification of Fig. 2 used for automatic gain control,

Fig. 12 shows an arrangement for securing th AVC bias for the circuit of Fig. 11,

Fig. 13 illustrates a modification of Fig. 11,

Fig. 14 shows the method of securing AVC bias for the circuit of Fig. 13.

For the sake of simplicity in description theoretical and simplified circuits in accordance with the invention will first be described. In the description of these circuits triodes will be re- 5 ferred to and it will be assumed that the gain control is manual. However, in practice valves other than triodes may be usednormally high frequency pentodes will be usedand automatic gain control may be employed by substituting for ordinary manually variable resistances, resistances Whose value is automatically varied.

In one simple circuit arrangement in accordance with this invention and illustrated in the accompanying Figure 1 input voltage to be amplified is applied between the control grid G of a triode V and the negative terminal of the anode source AB therefor. The anode A of the triode is connected through an output resistance OR (from across which output potentials are taken) to the positive terminal of the source AB and the cathode C is connected to the negative terminal of the said source AB through a variable resistance CR which acts as a negative feedback impedance. With this arrangement if the value of the feed-back resistance CR be increased the gain will be decreased and the rectilinearity of the relationship between anode current and grid voltage will be increased.

This type of circuit may be extended to multivalve circuits with a feed-back resistance common to a number of valves, i. e. in the cathode lead common to more than one valve. For ex-. ample, as illustrated in Figure 2, input potentials may be applied between the control grid GI of the first valve VI and the negative terminal of a source (not shown) of anode potential. The anode Al of the first valve is coupled to the control grid G2 of a second valve V2 whose anode A2 is in turn coupled to the control grid G3 of a third valve V3. The anode circuit of each valve contains an anode resistance ORI, 0R2, 0R3 output potential being taken from the anode resistance 0R3 of the last valve V3. The cathodes CI,- C3 of the first and third valves VI V3 are connected directly together and through a variable resistance CR to the cathode C2 of the second valve which is connected to the negative terminal of the source of anode potential. In'order to simplify the figure the operating potential sources are not shown. With this. arrangement increases in the value of the variable resistance CR. will again decrease the gain and increase the rectilinearity of response.

In a still further arrangement illustrated in Figure 3 also employing three valves, VI, V2, V3 coupled in cascade a variable resistance CRI or CR3 is inserted in the cathode lead of each of first and third valves VI, V3 (i. e. between the cathode of each valve and the negative terminal of the source (not shown) of anode potential) and the cathodes CI, C3 of these valves are connected together not directly as in the preceding example, but through a second variable resistance RX. The cathode C2 of the second valve V2 is connected to the negative terminal of the source of anode potential as before. This may be termed a three valve cross coupled arrangement. With this arrangement increase of either or both of the variable resistances CRI', CR3 in the cathode leads of the first and third valves VI, V3 and/or decrease of the Variable resistance RX in the cross connection between the cathodes CI, C3 of the first and third valves will decrease the gain and increase the rectilinearity of response.

In a two valve cross coupled arrangement illustrated in Figure 4 input potentials are applied across a grid input load resistance as shown to the control grid GI of the first valve VI and the output is taken from across an anode resistance 0R2 in the anode circuit of the second valve V2 the valves being in cascade. ance CR2 is included in the cathode lead of the second valve V2 and a second variable resistance Rx is connected between the control grid GI of A variable resist the first valve VI and the cathode C2 of the sec ond valve V2. With this arrangement increase in the value of the variable resistance CR2 and/or decrease of the value of the variable resistance RX will decrease gain and increase rectilinearity of response. Figures 3 and 4 like Figure 2 are simplified diagrams with the operating potential sources omitted.

The above arrangements involve what may be termed negative feed back control, but it is possible to employ positive feed back control. Figure 5 illustrates in simplified diagram form one arrangement in which such control is used. Here there are two valves VI, V2 coupled in cascade as before, and as before input potentials are applied to the control grid GI of the first valve and.

taken from across an anode resistance 0R2 in the anode circuit of the second valve V2. The two cathodes CI, C2 are connected directly together and through a common variable resistance CRI to the negative terminal of the source (not shown) of anode potential. With this arrangement increase in the value of the variable resistance CRI willincrease the gain but decrease the rectilinearity of response.

Both positive and negative feed back control may be employed in combination. One such arrangement employing three valves VI, V2, V3 coupled in cascade is illustrated in simplified diagram form in Figure 6. Here input potentials are applied between the control grid GI of the first valve VI and the far end of a cathode resistance CRI (this may be fixed) and output potentials are taken from across an anode resistance 0R3 in the anode circuit of the last valve V3. Variable resistances CR2 and CR3 are included in the cathode leads of the second and third valves V2, V3; a third variable resistance RXI is inserted between the cathode C2 of the second valve V2 and the cathode CI of the first valve VI and a fourth variable resistance RXZ is included between the cathode CI of the first valve VI and the cathode C3 of the third Valve V3. With this arrangement increases of gain can be obtained by increasing CR2 and/ or by increasing RX2 and/or by decreasing CR3 and/or by decreasing RXI.

In applying the above types of circuit in practice there are certain points which have to be considered. For example the cathode lead resistances employed to provide feed back effects must, so far as it is possible, offer pure ohmic resistance to the frequencies required tobe handled by the amplifier, but it is at the same time desirable that when the value is changed for gain control purposes no change should be made in the potential drop due to anode direct current between cathode and earth; in other words, there should be provided a by-pass of relatively low D. C. resistance for the anode current but of high impedance to frequencies to be handled by the amplifier. There are various different ways of accomplishing this result.

Figure '7, which is a detail diagrammatic view illustrates one way. Here the variable feed back resistance CR in a cathode lead is shunted by a choke CH of low D. C. resistance. This arrangement though satisfactory in many cases is not without its limitations for stray capacityin the case of indirectly heated cathode valves this will be mostly constituted by the capacity between the cathode and its heateris in shunt between the cathode C and earth and imposes limitations at the higher frequencies. Such stray capacity is represented in broken lines in Figure 7 by a condenser K. At lower frequencies there is a limitation. imposedby the fact that theimpedance of the choke CH tends to become comparable with the resistance in shunt therewith, thus-introducing a phase angle efi'ect. An improved result as respects this limitation can be obtained as illustrated in Figure 8 by shun-ting the choke OH with a condenser TK designed to make the chokecondenser combination resonate at about the centre of the operating frequency band. In determining the value of the choke CH and condenser TK regard should, of course, be had to stray self-capacity (shown in broken lines at K) in efiective shunt with the combination. The principal limitation of this arrangement is that the impedance of the capacity and shunt inductance must be high compared to that of the variable resistance CR providing a feed back efiect, if the circuit is not to introduce a phase angle.

A preferred arrangement when high frequencies are in question is that illustrated in Figure 9. Here the variable feed back resistance CR. in the cathode lead is shunted by a variable turnable parallel tuned circuit L, TK, TKI the variable capacity I'K in which is gang controlled with the other tuned circuits (not shown) of the receiver. The condenser TKI, which is part of the total capacity of the tuned circuit. is a small trimmer condenser for compensating for cathode-heater capacity to enable correct ganging of the tuning condenser TK with other tuning resistances in the receiver. A circuit of this type will permit for ordinary broadcast receiver frequencies, the use of a feed-back variable resistance CR of the value up to about 100,000 ohms, or even higher.

A variation of the last described arrangement is illustrated in Figure 10/ Here the variable feed back resistance CR is shunted by two resistances, IR, 2B, in series the far end of the resistance 2R most remote from the cathode being connected to the end of the variable feed back resistance CR remote from the cathode and to the negative terminal of the anode potential source (notshown) the junction point of the two series resistances IR, 2R, being earthed. The resistance IR nearest the cathode is shunted by a condenser IK consisting either wholly or in part of the stray capacity (cathodeheater capacity principally) and the other of the two resistances is shunted by. an inductance 2L.

The two series resistances IR, 23 are equal and each is chosen of a value equal to the square root of the quotient obtained by dividing the value of the inductance ZL'by the value of the effective capacity, this said value of resistance being equal at a given predetermined frequency to the reactance of the inductance 2L and to the reactance of the capacity. With this dimensioning of the component parts the whole circuit in shunt with the variable feed back resistance CR behaves as though it were a simple resistance of value equal to one of the series resistances. The limitation of this type of arrangement is that the negative terminal of the anode potential source is not earthed and stray capacity to earth from this point must be lower than the cathode-heater capacity, while if large values of effective resistance are required for the whole circuit in shunt with the feed back resistance the value of inductance which must be used be- 7 comes rather unduly large.

In the preoedingspecific description variation of the resistance for manual control has been assumed and in high quality receivers this would no doubt be desirable either alone or in addition' to automatic variation. A so-calledlocaldistance switch'could be arranged to out out portions of the feed-back resistances as required.

Further, full 'automaticgain control in accordance with this invention may be obtained by employingas feed back resistances, resistances capable of direct electrical control e.' g. resistances whose value alters when a heater current passedtherethrough is altered. Such resistances are known and the necessary varying heater current for automatic gain control could be obtained as in the usual way by amplifying received signals to give a uni-directional varying current of value dependent thereon. There are also available as varying resistances for the purpose of this invention materials such as that known under the registered trade mark as thyrite whose resistance is a function of the current passed through it. It is also possible to employ dry rectifiers such as the well known copper oxide rectifier or electron devices such as diodes, these rectifier devices being, of course, operated over the appropriate part of their characteristics to give the required changes with current passage therethrough. Again variable resistances of the carbon pile type,'i. e. resistances whose value can be varied by varying mechanical pressure might be used the mechanical pressure being varied automatically by electro-magnetic means'ener gised in accordance with the received signal strength.

Where automatic varying resistances are employed in carrying out this invention precautions should be taken to ensure that distortions or cross-modulation are not introduced due tocurvature of the characteristic of the said resistances, e. g. where a diode is employed care should be taken' in design to ensure the avoidance of sweeping the diode bythe A. C. signal'irom a non-conducting to a conducting portion of its characteristic. Curvature of characteristic on the part of a variable feedback resistance employed in carrying out this invention may, however, be turned to advantage and employed'to compensate'for curvature of the characteristic of a valve whose gain is to be controlled, and it can be arranged for the compensation to be substantially perfect at some definite signal level. Though no doubt, the compensation will not remain perfect over a complete control range. it can usually be arranged to be sufficiently 'nearly perfect for an important part of the range. The curvature of the characteristic-of a rectifier eniployed as a feed-back resistance in-carrying out this invention cantbe adjusted; as known per so, by providing adjustable resistances in parallel and/or in series with the rectifier. v

A simpleiwayof obtaining automatic gain control using a' rectifier as a variable feed back impedance for a high frequency amplifier Valve V! is illustrated in Figure 11. Here the cathode Ci of the said valve is connected through a diode or copper. oxide rectifier or similar device D'a'nd then through a resistance'AVR and lead AVC to a source (not shown) of automatic gain controlling potentials: This cathode is connected directly to the cathode C3 of a succeeding valve V3, for example as shown, the next valve but one in the cascadechain, the cathode C3 of which is connected through a network CR3 consisting of an inductance 3L resistance SR and capacity 30 all in shunt; and thenthrough a capacity shunted bias resistance. combination BR! to the negative terminal -H.T. ofthe source (not shown) of anode potential. The cathode C2 of the secondvalve V2 in the cascade chain is connected to the said negative terminal through a capacity shunted resistance combination BB2. The source of automatic volume control potential is arranged as known per se and so that in the absence of received signals it supplies a potential which is positivewith relation to the cathode C3 of the third valve V3, the potential supplied changing increasingly in the negative direction as the signal strength increases. The resistance AVR is shunted by a pair of condensers SCI, SC2, in series the junction point of these condensers being connected to earth and to the negative high tension terminal. Where a diode is employed it may be constituted as part of one of the valves whose gain is to be controlled, for example its cathode may be the cathode of the third valve V3 and its anode may be an auxiliary anode provided in that valve. This circuit illustrates the application of the method of Figure 2.

The automatic gain controlling potential for the arrangement of Figure 11. may be obtained in a variety of different ways. For example, as illustrated in Figure 12, the second detector of the receiverassumed to be a superheterodyne receivermay be a diode triode DT having its diode anode DA connected to the cathode DC through the input (intermediate frequency) tuned circuit IF in series with a capacity shunted resistance combination DBR, one end of the resistance in this combination being connected to the cathode DC and the other end to the control grid, DG as well as to the tuned circuit IF.

The cathode DC is connected to the negative terminal HT of the source (not shown) of anode potential through a capacity shunted resistance AVR'. With this arrangement the said cathode DC may be used as an automatic volume 40 control point, for it will be seen that in the absence of signals the said cathode will be positive due to the drop of potentials down the resistance AVR while with increased signal strength the grid DG will become increasingly 45 negative thus reducing the drop of potential down the said resistance. The lead AVC leads to the diode D of Figure 11 through the filter circuit constituted by the circuit elements SC2, AVR, and SCI.

In Fig. 11 the diode D functions as a device to vary the A. C. potential across network CR3. In other words, diode D acts as the variable tap along resistor CR of Fig. 2. This can readily be seen by observing that the impedance of diode D is connected in parallel with the network CR3. As signal strengthen increases the AVC bias becomes less positive (see Fig. 12) This results in an increase of the impedance of diode D. The alternating voltage across CR3, therefore, increases. This results in an increase in the degenerative feedback to amplifiers V1 and V3.

Figure '13 illustrates an arrangement in which ordinary well known variable mu control and feed back gain control in accordance with this invention are both employed. Here the cathode CI of the first valve VI of a radio receiver is connected via lead AVC to an automatic volume control point arranged to become increasingly positive with increased signal strength and is also connected through a capacity shunted resistance CRI to the negative terminal of a source of anode potential. A tapping point TCR upon this variable resistance is connected through a variable tunable parallel tuned circuit ATC3 to the cathode C3 of the next valve but one in the cascade chain of valves (valve V3) this tapping point being connected to the negative terminal of the source of anode potential through a bypasscondenser BPC. The said tapping point is also connected to the cathode C2 of the second valve V2 in the chain of valves and the grid G2 of this second valve is coupled, as in the usual way, to the anode circuit of the first valve, said anode circuit containing a parallel tunable circuit ATCI. The grid circuit of the first valve VI also contains a parallel tunable circuit GTCI as also does the anode circuit of the second valve V2 and the anode circuit of the third valve V3, these two tuned circuits being marked ATC2 and ATC3 respectively. The grid G3 of the third valve V3 is coupled to the anode of the second valve V2 as in the usual way, and the grid G2 of the second valve V2 is connected to the cathode C3 of the third through a diode ID the anode of which is directly connected to the cathode C3 and which is shunted by a resistance DR. The cathode C3 of the said third valve V3 is connected to the cathode of a diode 2D whose anode is connected to a tapping point T2 upon a resistance MR, one end of which is connected to the negative terminal HT of the anode potential source and the other end of which is connected to a positive point +HT. All the tuned circuits are gang controlled as indicatedin chain lines. As will be apparent the method of feed back illustrated is that of Figure 4. l

The automatic gainv control point may be cc stituted by any suitable point. in the receiver which becomes increasingly positive with increased signal strength; for example, if the receiver is a superheterodyne receiver the said gain control point may be constituted as illustrated in Figure 14 by the cathode DC of the second detector DT of saidreceiver, this second detector being an anode-bend detectorhaving its intermediate frequency input circuit IF connected between the control grid DG and. the negative terminal -HT of the source of anode potential.

which negative terminal is connected to the cathode through a capacity shunted resistance (not shown).

The diode 2D acts to vary the alternating po-- tential developed across cathode impedance ATC3. In other words, diode 2D cooperates in parallel with ATC3 to provide the variable impedance CR2 of Fig. 4. As the cathode of diode 2D is made more positive the impedance thereof increases; and the A. C. dropacross ATC3 increases with the result that the' degenerative feedback to amplifier V3 increases. This follows from the fact that the AVG bias becomes increasingly positive (see Fig. 14) as signal strength increases. Since the cathode of amplifier V3 (also V1 and V2) is connected to point TCR, it becomes more positive relative to grid G3 as signal strength increases. The cathode of 2D is tied to the cathode of V3; hence the impedance of diode 2D varies directly with the signal strength. Diode ID functions as variable impedance RX of Fig. 4. As the cathode C3 becomes more positive, the anode of ID becomes more positive. This reduces the impedance of the diode and permits increase of degenerative feedback to grid G2. In this way all three amplifiers V1V2V3 are reduced in gain, as AVC bias becomes more positive, by virtue of reduction in amplification of eachtube, as Well as by increase of negative feedback to amplifiers V2 and V3.

Having now particularly described and ascertained the nature of my said invention and in what manner the same is to be performed I declare that What I claim is:

1. In a signal transmission system, at least one transmission tube having a cathode, signal grid and output electrode, an impedance in the space current path of the tube connected between cathode and a point of relatively fixed potential and developing thereacross signal Voltage, a signal input circuit between said grid and said point whereby said signal voltage is degenerativelyapplied to said grid, an output circuit coupled to said output electrode, an electronic device of variable impedance in shunt with the said impedance for controlling the magnitude of said developed signal voltage, and means responsive to a direct current voltage derived from signals for controlling said variable impedance device in magnitude.

2. In a signal transmission system, at least one transmission tube having a cathode, signal grid and output electrode, an impedance in the space current path of the tube connected between cathode and a point of relatively fixed potential and developing thereacross signal voltage, a signal input circuit between said grid and said point whereby said signal voltage is degeneratively applied to said grid, an output circuit coupled to said output electrode, a device of variable impedance operatively associated with the said impedance for controlling the magnitude of said signal voltage, and means for controlling said variable impedance device in magnitude, said device comprising a diode connected in parallel with said impedance.

3. In a signal transmission system, at least one transmission tube having a cathode, signal grid and output electrode, an impedance in the space current path of the tube connected between cathode and a point of relatively fixed potential and developing thereacross signal voltage, a signal input circuit between said grid and said point whereby said signal voltage is degeneratively applied to said grid, an output circuit coupled to said output electrode, an electronic device of variable impedance connected in parallel with the said impedance for controlling the magnitude of said signal voltage, and-means for controlling said variable impedance device in magnitude, said impedance comprising a resonant circuit tuned to the same frequency as the signal input circuit.

4. In a signal transmission system, at least one transmission tube having a cathode, signal grid and output electrode, an impedance in the space current path of the tube connected between cathode and a point of relatively fixed potential and developing thereacross signal voltage, a signal input circuit between said grid and said point whereby said signal voltage is degeneratively applied to said grid, an output circuit coupled to said output electrode, a diode device of variable impedance in parallel with the said impedance for controlling the magnitude of said signal voltage, and means responsive to a direct current voltage derived from signals for controlling said variable impedance device in magnitude, an additional signal transmission tube in said system whose output electrodes are coupled to said signal input circuit, and means coupling an input electrode of said additional tube to said impedance thereby to provide degenerative feedback of said signal voltage thereto.

5. In a signal transmission system, at least one transmission tube having a cathode, signal grid and output electrode, an impedance in the space current path of the tube connected between cathode and a point of relatively fixed potential and developing thereacross signal voltage, a signal input circuit between said grid and said point whereby said signal voltage is degeneratively applied to said grid, an output circuit coupled to said output electrode, a diode device of variable impedance in parallel with the said impedance for controlling the magnitude of said signal voltage, and means for controlling said variable impedance device in magnitude, and said last means being responsive to direct current voltage derived from signal amplitude variation and simultaneously varying the direct current potential difference between the grid and cathode of said transmission tube.

6. In a signal receiver embodying at least two signal transmission tubes arranged in cascade, an impedance in the space current path of the second tube developing signal voltage thereacross, means impressing said signal voltage between the input electrodes of the second tube in degenerative phase, means comprising a diode in shunt with said impedance and responsive to direct current voltage derived from signal amplitude variation for controlling the said signal voltage magnitude, and means for impressing said signal voltage between the input electrodes of the first tube.

'7. In a signal receiver embodying at least two signal transmission tubes arranged in cascade, an impedance in the space current path of the second tube developing signal voltage thereacross, means impressing said signal voltage between the input electrodes of the second tube in degenerative phase, means comprising a diode in shunt witlisaid impedance and responsive to direct for impressing said signal voltage between the,

input electrodes of the first tube, said last impression being in degenerative phase, and said responsive means controlling said second diode.

8. In a signal transmission tube having input and output circuitseach tuned to a desired signal frequency, a circuit in the space current path of said tube tuned to said desired frequency and developing signal voltage which is applied to the tube in degenerative phase, an electronic device in parallel with said last circuit, means deriving a unidirectional control voltage from the desired signals, and means responsive to said control voltage for varying the conductivity of said electronic device thereby adjusting the magnitude of the degeneratively applied voltage.

9. In a signal transmission tube having input and output circuits each tuned to a desired signal frequency, a circuit in the space current path of said tube tuned to said desired frequency and developing signal voltage which is applied to the tube in degenerative phase, a diode in shunt with said last circuit, means deriving a unidirectional control voltage from the desired signals, means responsive to said control voltage for varying the diode conductively thereby adjusting the magnitude of the degeneratively applied voltage, and means for controlling the gain of said tube with said control voltage. r

, NOEL MEYER RUST. 

